CEMENT
General:
The
history of cementing material is as old as the history of engineering
construction. Some kind of cementing material were used by Egyptians romans and
Indians in their ancient construction. It is believed that the early Egyptians
mostly used cementing materials, obtained by burning gypsum. Not much light has
been thrown on cementing material, used in the construction of the cites of
Harappa and Mohenjo-Daro.
An
analysis of mortar from the great Pyramid showed that it contained 81.5 per
cent calcium sulphate and only 9.5 per cent carbonate. The early Greeks and
Romans used carbonate. The remarkable obtained by burning limestone. The
remarkable hardness of the mortar used in the early roman brickwork. Some of
which still exit, Is presenting sufficient evidence of the perfection which the
art of cementing material had attained in ancient times, the superiority of
roman mortar has been attributed to thoroughness of mixing and long continued
ramming.
The
Greeks and Romans later became aware the face the certain volcanic ash and
tuff, when mixed with lime and sand yielded mortar possessing superior strength
and better durability in fresh or salt water. Roman builders used volcanic tuff
found near Pozzuoli village near mount Vesuvius in Italy. This volcanic tuff or
ash mostly siliceous in nature thus acquired the name pozzolana. Later on, the
name pozzolona was applied to any other material natural or artificial, having
nearly the same composition as that of volcanic tuff or ash found at Pozzuoli,
the Romans, in the absence of natural volcanic ash, used powered tiles or
pottery as pozzolana. In India. Powered brick named surkhi has been used in
mortar. The Indian practice of through mixing and long continued ramming of
lime mortar with or without the addition of surkhi yielded strong and
impervious mortar which confirmed the secret of superiority of Roman mortar.
It
is learnt that the Romans added blood, milk and lard to their mortar and
concrete to achieve better workability. Hemoglobin in a powerful air-entraining
agent and plasticizer which perhaps is yet another reason for the durability of
Roman structure. Probably they did not know about the durability aspect but
used them a workability agents. The cementing material made by Romans using
lime and natural or artificial pozzolana retained its position as the chief
building material for all work particularly. For hydraulic construction belabor
a principal authority in hydraulic
construction, recommended an intimate mixture of tiles, stone chips and scales
from black smith’s forge, carefully ground wadhed free coal and drit dried and
sifted and them mixed with fresh slaked lime for making good concrete .
When
we come to more recent times, the most important advance in the knowledge of
cements, the forerunner to the discoveries and manufacture of all modern
cements is undoubtedly the investigations carried out by John Smeaton. When he
was called upon to rebuild the Eddystone Light-house in 1756, he made extensive
enquiries into the state of art existing in those days and also conducted
experiments with a view to find out the best material to withstand the severe
action of sea water. Finally, he concluded that lime-stones which contained
considerable proportion of clayey matter yielded better lime possessing
superior hydraulic properties. In spite of the success of Smeaton’s experiments,
the use of hydraulic lime made little progress, and the old practice of mixture
of lime and pozzolana remained popular for a long period. In 1976 hydraulic
cement was made by calcining nodules of argillaceous lime-stones. In about 1800
the product thus obtained was called Roman cement. This type of cement was in
use till about 1850 after which this was outdated by portland cement.
Early
History of Modern Cement:
The
investigations of L.J. Vicat led him to
prepare an artificial hydraulic lime by calcining an intimate mixture of
limestone and clay. This process may be regarded as the leading knowledge to
the manufacture of Portland cement. James Frost also patented a cement of this
kind Joseph Aspdin’s first cement works, around 1823, at Kirkgate in in 1811
and established The story of the invention of Portland cement is, however,
attributed to Joseph Aspdin, a Leeds builder and bricklayer, even though
similar procedures had been adopted by other inventors. Joseph Aspdin took the patent of portland cement
on 21st October 1824. The fancy name of portland was given owing to the
resemblance of this hardened cement to the natural stone occurring at Portland
in England. In his process Aspdin mixed and ground hard limestones and finely
divided clay into the form of slurry and calcined it in a furnace similar to a
lime kiln till the CO2 was expelled. The mixture so calcined was then ground to
a fine powder. Perhaps, a temperature lower than the clinkering temperature was
used by Aspdin. Later in 1845 Isaac Charles Johnson burnt a mixture of clay and
chalk till the clinkering stage to make better cement and established factories
in 1851. In the early period, cement was
used for making mortar only. Later the use of cement was extended for making
concrete. As the use of Portland cement was increased for making concrete,
engineers called for consistently higher
standard material for use in major works. Association of Engineers,
Consumers and Cement Courtesy : Ambuja Technical Literature
Manufacturers
have been established to specify standards for cement. The German standard
specification for Portland cement was drawn in 1877. The British standard
specification was first drawn up in 1904. The first ASTM specification was
issued in 1904.
In
India, Portland cement was first manufactured in 1904 near Madras, by the South
India Industrial Ltd. But this venture failed. Between 1912 and 1913, the
Indian Cement Co. Ltd., was established at Porbander (Gujarat) and by 1914 this
Company was able to deliver about 1000 tons of Portland cement. By 1918 three
factories were established. Together they were able to produce about 85000 tons
of cement per year. During the First Five-Year Plan (1951-1956) cement
production in India rose from 2.69 million tons to 4.60 million tons. By 1969
the
total production of cement in India was 13.2 million tons and India was then
occupying the 9th place in the world, with the USSR producing 89.4 million
tonnes and the USA producing 70.5 million tonnes1.1. Table 1.1 shows the Growth
of Cement Industry through Plans.
Prior
to the manufacture of Portland cement in India, it was imported from UK and
only a few reinforced concrete structures were built with imported cement. A
three storeyed structure built at Byculla, Bombay is one of the oldest RCC
structures using Portland cement in India. A concrete masonry building on Mount
Road at Madras (1903), the har-ki-pahari bridge at Haridwar (1908) and the
Cotton Depot Bombay, then one of the largest of its kind in the world (1922)
are some of the oldest concrete structures in India
The
perusal of table 1.2 shows that per capita cement consumption in India is much
less than world average. Considerable infrastructural development is needed to
build modern India. Production of more cement, knowledge and economical
utilisation of cement is the need of the day.
The
early scientific study of cements did not reveal much about the chemical
reactions that take place at the time burning. A deeper study of the fact that
the clayey constituents of limestone are responsible for the hydraulic
properties in lime (as established by John Smeaton) was not taken for further
research. It may be mentioned that among the earlier cement technologists,
Vicat, Le Chatelier and Michaelis were the pioneers in the theoretical and
practical field.
Systematic
work on the composition and chemical reaction of Portland cement was first
begun in the United States. The study on setting was undertaken by the Bureau
of Standards and since 1926 much work on the study of Portland cement was also
conducted by the Portland Cement Association, U.K. By this time, the
manufacture and use of Portland cement had spread to many countries. Scientific
work on cements and fundamental contributions to the chemistry of Portland
cements were carried out in Germany, Italy, France, Sweden, Canada and USSR, in
addition to Britain and USA. In Great Britain with the establishment of
Building Research Station in 1921 a systematic research programme was
undertaken and many major contributions have been made. Early literatures on
the development and use of Portland cements may be found in the Building
Science Abstracts published by Building Research Station U.K. since 1928,
“Documentation Bibliographique” issued quarterly since 1948 in France and
“Handbuch der Zement Literature” in Germany.
Manufacture
of Portland Cement
The
raw materials required for manufacture of Portland cement are calcareous
materials, such as limestone or chalk, and argillaceous material such as shale
or clay. Cement factories are established where these raw materials are
available in plenty. Cement factories have come up in many regions in India,
eliminating the inconvenience of long distance transportation of raw and
finished materials.
The
process of manufacture of cement consists of grinding the raw materials, mixing
them intimately in certain proportions depending upon their purity and
composition and burning them in a kiln at a temperature of about 1300 to
1500°C, at which temperature, the material sinters and partially fuses to form
nodular shaped clinker. The clinker is cooled and ground to fine powder with
addition of about 3 to 5% of gypsum. The product formed by using this procedure
is Portland cement.
There
are two processes known as “wet” and “dry” processes depending upon whether the
mixing and grinding of raw materials is done in wet or dry conditions. With a
little change in the above process we have the semi-dry process also where the
raw materials are ground dry and then mixed with about 10-14 per cent of water
and further burnt to clinkering temperature.
For
many years the wet process remained popular because of the possibility of more
accurate control in the mixing of raw materials. The techniques of intimate
mixing of raw materials in powder form was not available then. Later, the dry
process gained momentum with the modern development of the technique of dry
mixing of powdered materials using compressed air. The dry process requires
much less fuel as the materials are already in a dry state, whereas in the wet process
the slurry contains about 35 to 50 per cent water. To dry 6 " Concrete Technology the
slurry we thus require more fuel. In India most of the cement factories used
the wet process. Recently a number of factories have been commissioned to
employ the dry process method. Within next few years most of the cement
factories will adopt dry process system.
In
the wet process, the limestone brought from the quarries is first crushed to
smaller fragments. Then it is taken to a ball or tube mill where it is mixed
with clay or shale as the case may be and ground to a fine consistency of
slurry with the addition of water. The slurry is a liquid of creamy consistency
with water content of about 35 to 50 per cent, wherein particles, crushed to
the fineness of Indian Standard Sieve number 9, are held in suspension. The
slurry is pumped to slurry tanks or basins where it is kept in an agitated
condition by means of rotating arms with chains or blowing compressed air from
the bottom to prevent settling of limestone and clay particles. The composition
of the slurry is tested to give the required chemical composition and corrected
periodically in the tube mill and also in the slurry tank by blending slurry
from different storage tanks. Finally, the corrected slurry is stored in the
final storage tanks and kept in a homogeneous condition by the agitation of
slurry.
The
corrected slurry is sprayed on to the upper end of a rotary kiln against hot
heavy hanging chains. The rotary kiln is an important component of a cement
factory. It is a thick steel cylinder of diameter anything from 3 metres to 8
metres, lined with refractory materials, mounted on roller bearings and capable
of rotating about its own axis at a specified speed.
The
length of the rotary kiln may vary anything from 30 metres to 200 metres. The
slurry on being sprayed against a hot surface of flexible chain loses moisture
and becomes flakes. These flakes peel off and fall on the floor. The rotation
of the rotary kiln causes the flakes to move from the upper end towards the
lower end of the kiln subjecting itself to higher and higher temperature. The
kiln is fired from the lower end. The fuel is either powered coal, oil or
natural gass. By the time the material rolls down to the lower end of the
rotary kiln, the dry material
Concrete
Technology A view of Limestone quarry, raw material preparation : The prime raw
material limestone after blasting in mines is broken into big boulders. Then it
is transported by dumpers, tippers to limestone crusher where it is crushed to
15 to 20 mm size
STACKER
FOR CRUSHED LIMESTONE
RECLAIMER FOR CRUSHED LIMESTONE
After
crushing, the crushed limestone is piled longitudinally by an equipment called
stacker. The stacker deposits limestone
longitudinally in the form of a pile. The pile is normally 250 to 300 m long
and 8-10 m height. The declaimer cuts the pile vertically, simultaneously from
top to bottom to ensure homogenization of limestone. Reclaimer for homogenization of crushed
limestone.
undergoes
a series of chemical reactions until finally, in the hottest part of the kiln,
where the temperature is in the order of 1500°C, about 20 to 30 per cent of the
materials get fused.
Lime,
silica and alumina get recombined. The fused mass turns into nodular form of
size 3 mm to 20 mm known as clinker. The clinker drops into a rotary cooler
where it is cooled under controlled conditions The clinker is stored in silos
or bins. The clinker weighs about 1100 to 1300 gms per litre. The litre weight
of clinker indicates the quality of clinker.
The
cooled clinker is then ground in a ball mill with the addition of 3 to 5 per cent
of gypsum in order to prevent flash-setting of the cement. A ball mill consists
of several compartments charged with progressively smaller hardened steel
balls. The particles crushed to the required fineness are separated by currents
of air and taken to storage silos from where the cement is bagged or filled
into barrels for bulk supply to dams or other large work sites.
In
the modern process of grinding, the particle size distribution of cement
particles are maintained in such a way as to give desirable grading pattern.
Just as the good grading of aggregates is essential for making good concrete,
it is now recognised that good grading pattern of the cement particles is also
important.
Dry
Process:
In
the dry and semi-dry process the raw materials are crushed dry and fed in
correct proportions into a grinding mill where they are dried and reduced to a
very fine powder. The dry powder called the raw meal is then further blended
and corrected for its right composition and mixed by means of compressed air.
The aerated powder tends to behave almost like liquid and in about one hour of
aeration a uniform mixture is obtained.
The
blended meal is further sieved and fed into a rotating disc called granulator.
A quantity of water about 12 per cent by wright is added to make the blended
meal into pellets.
This
is done to permit air flow for exchange of heat for further chemical reactions
and conversion of the same into clinker further in the rotary kiln.
The
equipments used in the dry process kiln is comparatively smaller. The process
is quite economical. The total consumption of coal in this method is only about
100 kg when compared to the requirement of about 350 kg for producing a ton of
cement in the wet process. During March 1998, in India, there were 173 large
plants operating, out of which 49 plants used wet process, 115 plants used dry
process and 9 plants used semi-dry process.
Since
the time of partial liberalisation of cement industry in India (1982), there
has been an upgradation in the quality of cement. Many cement companies
upgraded their plants both in respect of capacity and quality. Many of the
recent plants employed the best equipments, such as cross belt analyser
manufactured by Gamma-Metrics of USA to find the composition of limestone at
the conveyor belts, high pressure twin roller press, six stage preheater,
precalciner and vertical roller mill. The latest process includes stacker and
reclaimer, on-line X-ray analyser, Fuzzy Logic kiln control system and other
modern process control. In one of the recently built cement plant at
Reddypalayam near Trichy, by Grasim Indistries, employed Robot for automatic
collection of hourly samples from 5 different places on the process line and help
analyse the ame, throughout 24 hours, untouched by men, to avoid human errors
in quality control. With all the above sophisticated equipments and controls,
consistent quality of clinker is produced.
The
methods are commonly employed for direct control of quality of clinker. The
first method involves reflected light optical microscopy of polished and etched
section of clinker
"
Concrete
Technology Close circuit grinding
technology is most modern grinding system for raw mix as well as for
clinker grinding. The systems are in compound mode and are equipped with high
efficiency Roller press and separators. The above mentioned system enables to
maintain low power consumption for grinding as well as Electronic packers: it has continuous narrow particle size distribution.
With this circuit, it is weighing system and it ensures that the possible to
manufacture higher surface area of product bags separating from the nozzles
have as per customers, requirement.
accurate weight of cement. The weight of filled bag is also displayed on
the packer. Multi-compartment silo.
Cross section of multi-compartment silo. Jumbo bag transportation. followed by point count of areas occupied by
various compounds. The second method, which is also applicable to powdered cement, involves X-ray
diffraction of powder specimen.
Calibration curves based on known mixtures of pure compounds, help to
estimate the compound composition. As a rough and ready method, litre weight
(bulk density) of clinker is made use of to ascertain the quality. A litre
weight of about 1200 gms. is found to be satisfactory. Jumbo bag packing. It is important to note that the strength
properties of cement are considerably influenced by the cooling rate of
clinker.
Concrete
Technology It can be seen from the table that a moderate rate of cooling of
clinker in the rotary cooler will result in higher strength. By moderate
cooling it is implied that from about 1200°C, the clinker is brought to about
500°C in about 15 minutes and from the 500°C the temperature is brought down to
normal atmospheric temperature in about 10 minutes.
The
rate of cooling influences the degree of crystallisation, the size of the
crystal and the amount of amorphous materials present in the clinker. The
properties of this amorphous material for similar chemical composition will be
different from the one which is crystallined.
Chemical
Composition:
The
raw materials used for the manufacture of cement consist mainly of lime,
silica, alumina and iron oxide. These oxides interact with one another in the
kiln at high temperature to form more complex compounds. The relative
proportions of these oxide compositions are responsible for influencing the
various properties of cement; in addition to rate of cooling and fineness of
grinding. Table 1.4 shows the approximate oxide composition limits of ordinary
Portland cement.
In
addition to the four major compounds, there are many minor compounds formed in
the kiln. The influence of these minor compounds on the properties of cement or
hydrated compounds is not significant. Two of the minor oxides namely K2O and
Na2O referred to as alkalis in cement are of some importance. This aspect will
be dealt with later when discussing alkali-aggregate reaction. The oxide
composition of typical Portland cement and the corresponding calculated
compound composition
Schematic presentation of various
compounds in clinker Courtesy : All the photographs on manufacture of cement
are by Grasim Industries Cement Division Tricalcium silicate and dicalcium
silicate are the most important compounds responsible for strength. Together
they constitute 70 to 80 per cent of cement. The average C3S content in modern
cement is about 45 per cent and that of C2S is about 25 per cent. The sum of
the contents of C3A and C4AF has decreased slightly in modern cements. The
calculated quantity of the compounds in cement varies greatly even for a
relatively small change in the oxide composition of the raw materials. To
manufacture a cement of stipulated compound composition, it becomes absolutely
necessary to closely control the oxide composition of the raw materials. An
increase in lime content beyond a certain value makes it difficult to combine
with other compounds and free lime will exist in the clinker which causes
unsoundness in cement. An increase in silica content at the expense of the
content of alumina and ferric oxide will make the cement difficult to fuse and
form clinker. Cements with a high total alumina and high ferric oxide content
is favourable to the production of high early strengths in cement. This is
perhaps due to the influence of these oxides for the complete combining of the
entire quantity of lime present to form tricalcium silicate.
The
advancement made in the various spheres of science and technology has helped us
to recognise and understand the micro structure of the cement compounds before
hydration and after hydration. The X-ray powder diffraction method, X-ray
fluorescence method and use of powerful electron microscope capable of
magnifying 50,000 times or even more has helped to reveal the crystalline or
amorphous structure of the unhydrated or hydrated cement.
Both
Le Chatelier and Tornebohm observed four different kinds of crystals in thin
sections of cement clinkers. Tornebohm called these four kinds of crystals as
Alite, Belite, Celite and Felite. Tornebohm’s description of the minerals in
cement was found to be similar to Bogue’s description of the compounds.
Therefore, Bogue’s compounds C3S, C2S, C3A and C4AF are sometimes called in
literature as Alite, Belite, Celite and Felite respectively.
Hydration
of Cement:
Anhydrous
cement does not bind fine and coarse aggregate. It acquires adhesive property
only when mixed with water. The chemical reactions that take place between
cement and water is referred as hydration of cement.
The
chemistry of concrete is essentially the chemistry of the reaction between
cement and water.On account of hydration certain products are formed. These
products are important because they have cementing or adhesive value. The quality,
quantity, continuity, stability and the
rate of formation of the hydration products are important. Anhydrous cement compounds when mixed with
water, react with each other to form hydrated compounds of very low solubility.
The hydration of cement can be visualized in two ways. The first is “through
solution” mechanism. In this the cement compounds dissolve to produce a
supersaturated solution from which different hydrated products get
precipitated. The second possibility is
"
Concrete
Technology that water attacks cement compounds in the solid state converting
the compounds into hydrated products starting from the surface and proceeding
to the interior of the compounds with time. It is probable that both “through
solution” and “solid state” types of mechanism may occur during the course of
reactions between cement and water. The former mechanism may predominate in the
early stages of hydration in view of large quantities of water being available,
and the latter mechanism may operate during the later stages of hydration.
Heat of
Hydration:
The
reaction of cement with water is exothermic. The reaction liberates a
considerable quantity of heat. This liberation of heat is called heat of
hydration. This is clearly seen if freshly mixed cement is put in a vaccum
flask and the temperature of the mass is read at intervals.
The
study and control of the heat of hydration becomes important in the
construction of concrete dams and other mass concrete constructions. It has
been observed that the temperature in the interior of large mass concrete is 50°C
above the original temperature of the concrete mass at the time of placing and
this high temperature is found to persist for a prolonged period. Fig 1.2 shows
the pattern of liberation of heat from setting cement1.4 and during early
hardening period.
On
mixing cement with water, a rapid heat evolution, lasting a few minutes,
occurs. This heat evolution is probably due to the reaction of solution of
aluminates and sulphates (ascending peak A). This initial heat evolution ceases
quickly when the solubility of aluminate is depressed by gypsum. (decending
peak A). Next heat evolution is on account of formation of ettringite and also
may be due to the reaction of C3S (ascending peak B). Refer Fig. 1.2.
Different
compounds hydrate at different rates and liberate different quantities of heat.
Fig.
1.3 shows the rate of hydration of pure compounds. Since retarders are added to
control the flash setting properties of C3A, actually the early heat of
hydration is mainly contributed from the hydration of C3S. Fineness of cement
also influences the rate of development of heat but not the total heat. The
total quantity of heat generated in the complete hydration will depend upon the
relative quantities of the major compounds present in a cement.
Analysis
of heat of hydration data of large number of cements, Verbec and Foster1.5
computed
heat evolution of four major compounds of cement. Table 1.7. shows the heats of
hydration of four compounds.
Since the heat of hydration of
cement is an additive property, it can be predicted from an expression of the
type
H
= aA + bB + cC + dD
Where
H represents the heat of hydration, A, B, C, and D are the percentage contents
of C3S, C2S, C3A and C4AF. and a, b, c and d are coefficients representing the
contribution of 1 per cent of the corresponding compound to the heat of
hydration.
Normal
cement generally produces 89-90 cal/g in 7 days and 90 to 100 cal/g in 28 days.
The
hydration process is not an instantaneous one. The reaction is faster in the
early period and continues idenfinitely at a decreasing rate. Complete
hydration cannot be obtained under a period of one year or more unless the
cement is very finely ground and reground with excess of water to expose fresh
surfaces at intervals. Otherwise, the product obtained shows unattacked cores
of tricalcium silicate surrounded by a layer of hydrated silicate, which being
relatively impervious to water, renders further attack slow. It has been
observed that after 28
days
of curing, cement grains have been found to have hydrated to a depth of only
4µ. It has also been observed that complete hydration under normal condition is
possible only for cement particles smaller than 50µ.
A
grain of cement may contain many crystals of C3S or others. The largest
crystals of C3S
or
C2S are about 40µ. An average size would be 15-20µ. It is probable that the C2S
crystals present in the surface of a cement grain may get hydrated and a more
reactive compound like C3S lying in the interior of a cement grain may not get
hydrated.
The
hydrated product of the cement compound in a grain of cement adheres firmly to
the unhydrated core in the grains of cement. That is to say unhydrated cement
left in a grain of cement will not reduce the strength of cement mortar or
concrete, as long as the products of hydration are well compacted. Abrams
obtained strength of the order of 280 MPa using mixes with a water/cement ratio
as low as 0.08. Essentially he has applied tremendous pressure to obtain proper
compaction of such a mixture. Owing to such a low water/cement ratio, hydration
must have been possible only at the surface of cement grains, and a
considerable portion of cement grains must have remained in an unhydrated
condition.
The
present day High Performance concrete is made with water cement ratio in the
region of 0.25 in which case it is possible that a considerable portion of
cement grain remains unhydrated in the core. Only surface hydration takes
place. The unhydrated core of cement grain can be deemed to work as very fine
aggregates in the whole system.
Calcium
Silicate Hydrates:
During
the course of reaction of C3S and C2S with water, calcium silicate hydrate,
abbreviated C-S-H and calcium hydroxide, Ca(OH)2 are formed. Calcium silicate
hydrates are the most important products. It is the essence that determines the
good properties of concrete.
"
Concrete
Technology It makes up 50-60 per cent of the volume of solids in a completely
hydrated cement paste.
The
fact that term C-S-H is hyphenated signifies that C-S-H is not a well defined
compound. The morphology of C-S-H shows
a poorly crystalline fibrous mass.
It was considered doubtful that the
product of hydration of both C3S and C2S results in the formation of the same
hydrated compound. But later on it was seen that ultimately the hydrates of C3S
and C2S will turn out to be the same. The following are the approximate
equations showing the reactions of C3S and C2S with water.
However, the simple equations given
above do not bring out the complexities of the actual reactions.
It
can be seen that C3S produces a comparatively lesser quantity of calcium
silicate hydrates and more quantity of Ca(OH)2 than that formed in the
hydration of C2S. Ca(OH)2 is not a desirable product in the concrete mass, it
is soluble in water and gets leached out making the concrete porous,
particularly in hydraulic structures. Under such conditions it is useful to use
cement with higher percentage of C2S content.
C3S
readily reacts with water and produces more heat of hydration. It is
responsible for early strength of concrete. A cement with more C3S content is
better for cold weather concreting. The quality and density of calcium silicate
hydrate formed out of C3S is slightly inferior to that formed by C2S. The early
strength of concrete is due to C3S.
C2S
hydrates rather slowly. It is responsible for the later strength of concrete.
It produces less heat of hydration. The calcium silicate hydrate formed is
rather dense and its specific surface is higher. In general, the quality of the
proudct of hydration of C2S is better than that produced in the hydration of
C3S. Fig 1.4 shows the development of strength of pure compounds.
Calcium
Hydroxide:
The
other products of hydration of C3S and C2S is calcium hydroxide. In contrast to
the C-S-H, the calcium hydroxide is a compound with a distinctive hexagonal
prism morphology.
It
constitutes 20 to 25 per cent of the volume of solids in the hydrated paste.
The lack of durability of concrete, is on account of the presence of calcium
hydroxide. The calcium hydroxide also reacts with sulphates present in soils or
water to form calcium sulphate which further reacts with C3A and cause
deterioration of concrete. This is known as sulphate attack.
To
reduce the quantity of Ca(OH)2 in concrete and to overcome its bad effects by
converting it into cementitious product is an advancement in concrete
technology. The use of blending Cement " 21
materials
such as fly ash, silica fume and such other pozzolanic materials are the steps
to overcome bad effect of Ca(OH)2 in concrete. This aspect will be dealt in
greater detail later.
The
only advantage is that Ca(OH)2, being alkaline in nature maintain pH value
around 13 in the concrete which resists the corrosion of reinforcements.
Calcium Aluminate Hydrates:
The
hydration of aluminates has been the subject of numerous investigations, but
there is still some uncertainty about some of the reported products. Due to the
hydration of C3A , a calcium aluminate system CaO – Al2O3 – H2O is formed. The
cubic compound C3 AH6 is probably the only stable compound formed which remains
stable upto about 225°C.
The
reaction of pure C3 A with water is very fast and this may lead to flash set.
To prevent this flash set, gypsum is
added at the time of grinding the cement clinker. The quantity of gypsum added
has a bearing on the quantity of C3 A present. The hydrated aluminates do not contribute
anything to the strength of concrete. On the other hand, their presence is harmful
to the durability of concrete particularly where the concrete is likely to be
attacked by sulphates. As it hydrates very fast it may contribute a little to
the early strength. On hydration, C4AF
is believed to form a system of the form CaO – Fe2O3 – H2O. A hydrated calcium
ferrite of the form C3FH6 is comparatively more stable. This hydrated product
also does not contribute anything to the strength. The hydrates of C4AF show a
comparatively higher resistance to the attack of sulphates than the hydrates of
calcium aluminate.
From
the standpoint of hydration, it is convenient to discuss C3A and C4AF together,
because the products formed in the presence of gypsum are similar. Gypsum and
alkalies go into solution quickly and the solubility of C3A is depressed.
Depending upon the concentration of aluminate and sulphate ions in solution,
the pricipitating crystalline product is either the calcium aluminate
trisulphate hydrate (C6A S 3H32) or calcium aluminate monosulhphate hydrate
(C4A S H18). The calcium aluminate trisulphate hydrate is known as ettringite.
Ettringite
is usually the first to hydrate and crystallise as short prismatic needle on
account of the high sulphate/aluminate ratio in solution phase during the first
hour of hydration. When sulphate in solution gets depleted, the aluminate
concentration goes up due to renewed hydration of C3A and C4AF. At this stage
ettringite becomes unstable and is gradually converted into mono-sulphate,
which is the final product of hydration of portland cements containing more
than 5 percent C3A.
The
amount of gypsum added has significant bearing on the quantity of aluminate in
the cement. The maintenance of aluminate-to-sulphate ratio balance the normal
setting 22 " Concrete Technology
behaviour of cement paste. The various setting phenomena affected by an
imbalance in the A/ S ratio is of practical significance in concrete
technology.
Many
theories have been put forward to explain what actually is formed in the
hydration of cement compounds with water. It has been said earliiar that
product consisting of (CaO.SiO2.H2O) and Ca(OH)2 are formed in the hydration of
calcium silicates. Ca(OH)2 is an unimportant product, and the really
significant product is (CaO.SiO2.H2O). For simplicity’s sake this product of
hydration is sometime called tobermorite gel because of its structural
similarity to a naturally occurring mineral tobermorite. But very commonly the
product of hydration is referred to as C – S – H gel.
It
may not be exactly correct to call the product of hydrations as gel. Le
chatelier identified the products as crystalline in nature and put forward his
crystalline theory. He explained that the precipitates resemble crystals
interlocked with each other. Later on Michaelis put forward his colloidal theory
wherein he considered the precipitates as colloidal mass, gelatinous in nature.
It is agreed that an element of truth exists in both these theories. It is
accepted now that the product of hydration is more like gel, consisting of
poorly formed, thin, fibrous crystals that are infinitely small. A variety of
transitional forms are also believed to exist and the whole is seen as bundle
of fibres, a fluffy mass with a refractive index of 1.5 to 1.55, increasing
with age.
Since
the gel consists of crystals, it is porous in nature. It is estimated that the
porosity of gel is to the extent of 28%. The gel pores are filled with water.
The pores are so small that the specific surface of cement gel is of the order
of 2 million sq. cm. per gm. of cement. The porosity of gel can be found out by
the capillary condensation method or by the mercury porosimetry method.
Structure
of Hydrated Cement:
To
understand the behaviour of concrete, it is necessary to acquaint ourselves
with the structure of hydrated hardened cement paste. If the concrete is
considered as two phase material, namely, the paste phase and the aggregate
phase, the understanding of the paste phase becomes more important as it
influences the behaviour of concrete to a much greater extent. It will be
discussed later that the strength, the permeability, the durability, the drying
shrinkage, the elastic properties, the creep and volume change properties of
concrete is greatly influenced by the paste structure. The aggregate phase
though important, has lesser influence on the properties of concrete than the
paste phase. Therefore, in our study to understand concrete, it is important
that we have a deep understanding of the structure of the hydrated hardened
cement paste at a phenomenological level.
Transition
Zone:
Concrete
is generally considered as two phase material i.e., paste phase and aggregates
phase. At macro level it is seen that aggregate particles are dispersed in a
matrix of cement paste. At the microscopic level, the complexities of the
concrete begin to show up, particularly in the vicinity of large aggregate
particles. This area can be considered as a third phase, the transition zone,
which represents the interfacial region between the particles of coarse
aggregate and hardened cement paste. Transition zone is generally a plane of
weakness and, therefore, has far greater influence on the mechanical behaviour
of concrete.
Although
transition zone is composed of same bulk cement paste, the quality of paste in
the transition zone is of poorer quality. Firstly due to internal bleeding,
water accumulate below elongated, flaky and large pieces of aggregates. This
reduces the bond between paste Cement " and aggregate in general. If we go into little
greater detail, the size and concentration of crystalline compounds such as
calcium hydroxide and ettringite are also larger in the transition zone. Such a
situation account for the lower strength of transition zone than bulk cement
paste in concrete.
Due
to drying shrinkage or temperature variation, the transition zone develops
microcracks even before a structures is loaded. When structure is loaded and at
high stress levels, these microcracks propagate and bigger chracks are formed
resulting in failure of bond.
Therefore,
transition zone, generally the weakest link of the chain, is considered
strength limiting phase in concrete. It is because of the presence of
transition zone that concrete fails at considerably lower stress level than the
strength of bulk paste or aggregate.
Sometimes
it may be necessary for us to look into the structure of hardening concrete
also. The rate and extent of hydration of cement have been investigated in the
past using a variety of techniques. The techniques used to study the structure
of cement paste include measurements of setting time, compressive strength, the
quantity of heat of hydration evolved, the optical and electron microscope
studies coupled with chemical analysis and thermal analysis of hydration
products. Continuous monitoring of reactions by X-ray diffractions and
conduction calorimetry has also been used for the study.
Measurements
of heat evolved during the exothermic reactions also gives valuable insight
into the nature of hydration reactions. Since approximately 50% of a total heat
The
mechanical properties of the hardened concrete depend more on the physical
structure of the products of hydration than on the chemical composition of the
cement. Mortar and concrete, shrinks and cracks, offers varying chemical
resistance to different situations, creeps in different magnitude, and in
short, exhibits complex behaviour under different conditions. Eventhough it is
difficult to explain the behaviour of concrete fully and exactly, it is
possible to explain the behaviour of concrete on better understanding of the
structure of the hardened cement paste. Just as it is necessary for doctors to
understand in great detail the anatomy of the human body to be able to diagnose
disease and treat the patient with medicine or surgery, it is necessary for concrete
technologists to fully understand the structure of hardened cement paste in
great detail to be able to appreciate and rectify the ills and defects of the
concrete.
hardening
paste consists of hydrates of various compounds, unhydrated cement particles
and water. With further lapse of time the quantity of unhydrated cement left in
the paste decreases and the hydrates of the various compounds increase. Some of
the mixing water is used up for chemical reaction, and some water occupies the
gel-pores and the remaining water remains in the paste. After a sufficiently
long time (say a month) the hydrated paste can be considered to be consisting
of about 85 to 90% of hydrates of the various compounds and 10 to 15 per cent
of unhydrated cement. The mixing water is partly used up in the chemical
reactions. Part of it occupies the gel-pores and the remaining water unwanted
for hydration or for filling in the gel-pores causes capillary cavities. These
capillary cavities may have been fully filled with water or partly with water
or may be fully empty depending upon the age and the ambient temperature and
humidity conditions. Figure 1.6 (a) and (b) schematically depict the structure
of hydrated cement paste. The dark portion represents gel. The small gap within
the dark portion represents gel-pores and big space such as marked “c”
represents capillary cavities.1.6 Fig. 1.7 represents the microscopic schematic
model of structure of hardened cement paste.
Water
Requirements for Hydration:
It
has been brought out earlier that C3S requires 24% of water by weight of cement
and C2S requires 21%. It has also been estimated that on an average 23% of
water by weight of cement is required for chemical reaction with Portland
cement compounds. This 23% of water chemically combines with cement and,
therefore, it is called bound water. A certain quantity of water is imbibed
within the gel-pores. This water is known as gel-water. It can be said that
bound water and gel-water are complimentary to each other. If the quantity of
water is inadequate to fill up the gel-pores, the formations of gel itself will
stop and if the formation of gel stops there is no question of gel-pores being
present. It has been further estimated that about 15 per cent by weight of
cement is required to fill up the gel-pores. Therefore, a total 38 per cent of
water by weight of cement is required for the complete chemical reactions and
to occupy the space within gel-pores. If water equal to 38 per cent by weight
of cement is Diagrammatic representation of the Hydration process and formation
of cement gel.
"
Concrete
Technology only used it can be noticed that the resultant paste will undergo
full hydration and no extra water will be available for the formation of
undesirable capillary cavities. On the other hand, if more than 38 per cent of
water is used, then the excess water will cause undesirable capillary cavities.
Therefore greater the water above the minimum required is used (38 per cent),
the more will be the undesirable capillary cavities. In all this it is assumed
that hydration is taking place in a sealed container, where moisture to and
from the paste does not take place.
It
can be seen that the capillary cavities become larger with increased
water/cement ratio.
With
lower w/c ratio the cement particles are closer together. With the progress of
hydration, when the volume of anhydrous cement increases, the product of
hydration also increases. The increase in the volume of gel due to complete
hydration could fill up the space earlier occupied by water upto a w/c ratio of
0.6 or so. If the w/c ratio is more than 0.7, the increase in volume of the
hydrated product would never be sufficient to fill up the voids created by
water. Such concrete would ever remain as porous mass. This is to say that gel
occupies more and more space, that once occupied by mixing water. It has been
estimated that the volume of gel would be about twice the volume of unhydrated
cement.
The
diagrammatic representation of progress of hydration is sown in Fig. 1.8. Fig.
1.8
(a)
represents the state of cement particles immediately when dispersed in an
aqueous solution. During the first few minutes, the reaction rate is rapid and
the calcium silicate hydrate forms a coating around the cement grains See Fig.
1.8 (b). As hydration proceeds, hydration products, including calcium hydroxide
are precipitated from the saturated solution and bridge the gap between the
cement grains, and the paste stiffens into its final shape, see Fig. 1.8
(c).
Further hyudration involving some complex form of diffusion process results in
further deposition of the cement gel at the expense of the unhydrated cement
and capillary pore-water Fig. 1.8 (d).
What
has been described briefly is the approximate structure of hardened cement
paste on account of the hydration of some of the major compounds. Very little
cognisance is taken of the product of hydration of the other major and minor
compounds in cement. The morphology of product of hydration and the study of
structure of hardened cement paste in its entirety is a subject of continued
research.
The
development of high voltage electron microscopy, combined with developments of
skill in making very thin sections is making possible high resolution
photography and diffractometry while at the same time reducing damage to the
specimen while under observation. The scanning electron provides stereographic
images and a detailed picture of structure of cement paste. These facilitate
further to understand aggregate cement bond, micro fracture and porosity of
cement gel.
CHAPTER
-2
TYPE
OF CEMENT AND TESTING OF CEMENT
I n the previous
chapter we have discussed various properties of Portland cement in
general. We have seen that cements
exhibit different properties and characteristics depending upon their chemical compositions.
By changing the fineness of grinding or the oxide composition, cement can
be made to exhibit different properties.
In the past continuous efforts were made to produce different kinds of cement, suitable for different
situations by changing oxide composition and fineness of grinding. With the
extensive use of cement, for widely varying conditions, the types of cement
that could be made only by varying the
relative proportions of the oxide compositions, were not found to be
sufficient. Recourses have been taken to add one or two more new materials,
known as additives, to the clinker at the time of grinding, or to the use of
entirely different basic raw materials in the manufacture of cement. The use of
additives, changing chemical composition, and use of different raw materials
have resulted in the availability of many types of cements
" Concrete
Technology to cater to the need of the construction industries for specific
purposes. In this chapter we shall deal with the properties and use of various
kinds of cement. These cements are classified as Portland cements and
non-Portland cements. The distinction is mainly based on the methods of
manufacture. The Portland and Non-Portland cements generally used are listed
below: Indian standard specification number is also given against these
elements.
TYPE
OF CEMENT:
(a) Ordinary Portland Cement
(i ) Ordinary Portland Cement 33
Grade– IS 269: 1989
(ii) Ordinary Portland Cement 43
Grade– IS 8112: 1989
(iii) Ordinary Portland Cement 53
Grade– IS 12269: 1987
(b) Rapid Hardening Cement – IS 8041: 1990
(c) Extra Rapid Hardening Cement
(d) Sulphate Resisting Cement– IS 12330: 1988
(e) Portland Slag Cement– IS 455: 1989
(f) Quick Setting Cement
(g) Super Sulphated Cement– IS 6909: 1990
(h) Low Heat Cement –
IS 12600: 1989
(j) Portland Pozzolana Cement
– IS 1489 (Part I) 1991 (fly ash
based)
– IS 1489 (Part II) 1991 (calcined
clay based)
(k) Air Entraining Cement
(l) Coloured Cement: White Cement – IS 8042: 1989
(m) Hydrophobic Cement – IS 8043: 1991
(n) Masonry Cement – IS 3466: 1988
(o) Expansive Cement
(p) Oil Well Cement – IS 8229: 1986
(q) Rediset Cement
(r) Concrete Sleeper grade Cement– IRS-T 40: 1985
(s) High
Alumina Cement– IS 6452: 1989
(t) Very High
Strength Cement
ASTM
Classification
Before we discuss the above cements,
for general information, it is necessary to see how Portland cement are
classified under the ASTM (American Society for Testing Materials) standards.
As per ASTM, cement is designated as Type I, Type II, Type III, Type IV, Type V
and other minor types like Type IS, Type IP and Type IA IIA and IIIA.
Type
I
For use in general concrete
construction where the special properties specified for Types II, III, IV and V
are not required (Ordinary Portland Cement).
Type
II
For use in general concrete
construction exposed to moderate sulphate action, or where moderate heat of
hydration is required.
Type
III
For use when high early strength is
required (Rapid Hardening Cement).
Type
IV
For use when low heat of hydration is
required (Low Heat Cement).
Type V
For use when high sulphate
resistance is required (Sulphate Resisting Cement). ASTM standard also have
cement of the type IS. This consist of an intimate and uniform blend of
Portland Cement of type I and fine granulated slag. The slag content is between
25 and 70 per cent of the weight of Portland Blast-Furnace Slag Cement.
Type
IP
This
consist of an intimate and uniform Cross Section of Multi-compartment Silo for
blend of Portland Cement (or Portland Blast storing different types of
cement. Furnace Slag Cement) and fine
pozzolana in Courtesy : Grasim Industries Cement Division which the pozzolana
content is between 15 and 40 per cent of the weight of the total cement.
Type
IA, IIA and IIIA
These are type I, II or III cement in
which air-entraining agent is interground where air-entrainment in concrete is
desired
Ordinary
Portland Cement
Ordinary
Portland cement (OPC) is by far the most important type of cement. All the
discussions that we have done in the previous chapter and most of the
discussions that are going to be done in the coming chapters relate to OPC.
Prior to 1987, there was only one grade of OPC which was governed by IS
269-1976. After 1987 higher grade cements were introduced in India. The OPC was
classified into three grades, namely 33 grade, 43 grade and 53 grade depending
upon the strength of the cement at 28 days when tested as per IS 4031-1988. If
the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if
the strength is not less than 43N/mm2, it is called 43 grade cement, and if the
strength is not less then 53 N/mm2, it is called 53 grade cement. But the
actual strength obtained by these cements at the factory are much higher than
the BIS specifications.
It
has been possible to upgrade the qualities of cement by using high quality
limestone, modern equipments, closer on line control of constituents,
maintaining better particle size distribution, finer grinding and better
packing. Generally use of high grade cements offer many advantages for making
stronger concrete. Although they are little costlier than low grade cement,
they offer 10-20% savings in cement consumption and also they offer many other
hidden benefits. One of the most important benefits is the faster rate of
development 30 " Concrete Technology of
strength. In the modern construction activities, higher grade cements have
become so popular that 33 grade cement is almost out of the market. Table 2.9
shows the grades of cement manufactured in various countries of the world.
The
manufacture of OPC is decreasing all over the world in view of the popularity
of blended cement on account of lower energy consumption, environmental
pollution, economic and other technical reasons. In advanced western countries
the use of OPC has come down to about 40 per cent of the total cement
production. In India for the year 1998-99 out of the total cement production
i.e., 79 million tons, the production of OPC in 57.00
million
tons i.e., 70%. The production of PPC is 16 million tone i.e., 19% and slag
cement is 8 million tons i.e., 10%. In the years to come the use of OPC may
still come down, but all the same the OPC will remain as an important type for
general construction.
The
detail testing methods of OPC is separately discribed at the end of this
chapter.
Rapid
Hardening Cement (IS 8041–1990)
This
cement is similar to ordinary Portland cement. As the name indicates it
develops strength rapidly and as such it may be more appropriate to call it as
high early strength cement. It is pointed out that rapid hardening cement which
develops higher rate of development of strength should not be confused with
quick-setting cement which only sets quickly. Rapid hardening cement develops
at the age of three days, the same strength as that is expected of ordinary
Portland cement at seven days.
The
rapid rate of development of strength is attributed to the higher fineness of
grinding (specific surface not less than 3250 sq. cm per gram) and higher C3S
and lower C2S content.
A
higher fineness of cement particles expose greater surface area for action of
water and also higher proportion of C3S results in quicker hydration.
Consequently, capid hardening cement gives out much greater heat of hydration
during the early period. Therefore, rapid hardening cement should not be used
in mass concrete construction.
The
use of rapid heading cement is recommended in the following situations: (a) In
pre-fabricated concrete construction.
(b
) Where formwork is required to be removed early for re-use elsewhere, (c )
Road repair works,
(d
) In cold weather concrete where the rapid rate of development of strength
reduces the vulnerability of concrete to the frost damage.
The
physical and chemical requirements of rapid hardening cement are shown in
Tables 2.5 and 2.6 respectively.
Extra
Rapid Hardening Cement:
Extra
rapid hardening cement is obtained by intergrinding calcium chloride with rapid
hardening Portland cement. The normal addition of calcium chloride should not
exceed 2 per cent by weight of the rapid hardening cement. It is necessary that
the concrete made by using extra rapid hardening cement should be transported,
placed and compacted and finished within about 20 minutes. It is also necessary
that this cement should not be stored for more than a month.
Extra
rapid hardening cement accelerates the setting and hardening process. A large
quantity of heat is evolved in a very short time after placing. The
acceleration of setting, hardening and evolution of this large quantity of heat
in the early period of hydration makes the cement very suitable for concreting
in cold weather, The strength of extra rapid hardening Types of Cement "
cement
is about 25 per cent higher than that of rapid hardening cement at one or two
days and 10–20 per cent higher at 7 days. The gain of strength will disappear
with age and at 90
days
the strength of extra rapid hardening cement or the ordinary portland cement
may be nearly the same.
There
is some evidence that there is small amount of initial corrosion of
reinforcement when extra rapid hardening cement is used, but in general, this
effect does not appear to be progressive and as such there is no harm in using
extra rapid hardening cement in reinforced concrete work. However, its use in
prestress concrete construction is prohibited.
In
Russia, the attempt has been made to obtain the extra rapid hardening property
by grinding the cement to a very fine degree to the extent of having a specific
surface between 5000 to 6000 sq. cm/gm. The size of most of the particles are
generally less than 3 microns2.1.
It
is found that this very finely ground cement is difficult to store as it is
liable to air-set. It is not a common cement and hence it is not covered by
Indian standard.
Sulphate
Resisting Cement (IS 12330–1988):
Ordinary
Portland cement is susceptible to the attack of sulphates, in particular to the
action of magnesium sulphate. Sulphates react both with the free calcium
hydroxide in set-cement to form calcium sulphate and with hydrate of calcium
aluminate to form calcium sulphoaluminate, the volume of which is approximately
227% of the volume of the original aluminates. Their expansion within the frame
work of hadened cement paste results in cracks and subsequent disruption. Solid
sulphate do not attack the cement compound. Sulphates in solution permeate into
hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and
even hydrated silicates.
The
above is known as sulphate attack. Sulphate attack is greatly accelerated if
accompanied by alternate wetting and drying which normally takes place in
marine structures in the zone of tidal variations.
To
remedy the sulphate attack, the use of cement with low C3A content is found to
be effective. Such cement with low C3 A and comparatively low C4AF content is
known as Sulphate Resisting Cement. In other words, this cement has a high
silicate content. The specification generally limits the C3A content to 5 per
cent.
Tetracalcium
Alumino Ferrite (C3AF) varies in Normal Portland Cement between to 6 to 12%.
Since it is often not feasible to reduce the Al2O3 content of the raw material,
Fe2O3 may be added to the mix so that the C4AF content increases at the expense
of C3A. IS code limits the total content of C4AF and C3A, as follows.
2C3A
+ C4AF should not exceed 25%.
In
many of its physical properties, sulphate resisting cement is similar to
ordinary Portland cement. The use of sulphate resisting cement is recommended
under the following conditions: (a ) Concrete to be used in marine condition;
(b
) Concrete to be used in foundation and basement, where soil is infested with
sulphates;
(c
) Concrete used for fabrication of pipes which are likely to be buried in marshy
region or sulphate bearing soils;
(d
) Concrete to be used in the construction of sewage treatment works.
Portland
Slag Cement (PSC) (IS 455–1989)
Portland
slag cement is obtained by mixing Portland cement clinker, gypsum and Concrete Technology granulated blast furnace
slag in suitable proportions and grinding the mixture to get a thorough and
intimate mixture between the constituents. It may also be manufactured by
separately grinding Portland cement clinker, gypsum and ground granulated blast
furnace slag and later mixing them intimately. The resultant product is a
cement which has physical properties similar to those of ordinary Portland
cement. In addition, it has low heat of hydration and is relatively better
resistant to chlorides, soils and water containing excessive amount of
sulphates or alkali metals, alumina and iron, as well as, to acidic waters, and
therefore, this can be used for marine works
with advantage. The manufacture of blast furnace slag
cement
has been developed primarily to utilize blast furnace slag, a waste product
from blast furnaces. The development of this type of cement has considerably
increased the total output of cement production in India and has, in addition, provided a scope for profitable
use for an otherwise waste product. During 98-99 India produced 10% slag cement
out of 79 million tons. The quantity of granulated slag mixed with portland
clinker will range from 25-65 per cent. In different countries this cement is
known in different names. The quantity of slag mixed also will vary from
country to country Schematic representation of production of the maximum being
upto 85 per cent. Early blast furnace slag.
strength is mainly due to the cement clinker
fraction and later
strength is that due to the slag fraction. Separate grinding is used as an easy
means of verying the slag clinker proportion in the finished cement to meet the
market demand. Recently, under Bombay Sewage disposal project at Bandra, they
have used 70%
ground
granulated blast furnace slag (GGBS) and 30% cement for making grout to fill up
the trench around precast sewer 3.5 m dia embedded 40 m below MSL.
Portland
blast furnace cement is similar to ordinary Portland cement with respect to
fineness, setting time, soundness and strength. It is generally recognised that
the rate of hardening of Portland blast furnace slag cement in mortar or
concrete is somewhat slower than that of ordinary Portland cement during the
first 28 days, but thereafter increases, so that at 12 months the strength becomes
close to or even exceeds those of Portland cement. The heat of hydration of
Portland blast furnace cement is lower than that of ordinary Portland cement.
So this cement can be used in mass concrete structures with advantage. However,
in cold weather the low heat of hydration of Portland blast furnace cement
coupled with moderately low rate of strength development, can lead to frost
damage.
Extensive
research shows that the presence of GGBS leads to the enhancement of the
intrinsic properties of the concrete both in fresh and hardened states. The
major advantages currently recognised are:
(a ) Reduced heat of hydration;
(b ) Refinement of pore structure;
(c ) Reduced permeability;
(d ) Increased resistance to chemical
attack.
It
is seen that in India when the Portland blast furnace slag cement was first
introduced it met with considerable suspicion and resistance by the users. This
is just because some manufacturers did not use the right quality of slag. It
has been pointed out that only glassy granulated slag could be used for the
manufacture of slag cement. Air-cooled crystallined slag cannot be used for
providing cementitious property. The slag which is used in the manufacture of
various slag cement is chilled very rapidly either by pouring it into a large
body of water or by subjecting the slag stream to jets of water, or of air and
water. The purpose is to cool the slag quickly so that crystallisation is
prevented and it solidifies as glass. The product is called granulated slag.
Only in this form the slag should be used for slag cement. It the slag prepared
in any other form is used, the required quality of the cement will not be
obtained.
Portland
slag cement exhibits very low diffusivity to chloride ions and such slag cement
gives better resistance to corrosion of steel reinforcement.
Application
of GGBS Concrete
In
recent years the use of GGBS concrete is well recognised. Combining GGBS and
OPC
at
mixer is treated as equivalent to factory made PSC. Concrete with different
properties can be made by varying the proportions of GGBS.
While
placing large pours of concrete it is vital to minimise the risk of early age thermal
cracking by controlling the rate of temperature rise. One of the accepted
methods is through the use of GGBS concrete containing 50% to 90% GGBS.
Generally, a combination of 70%
GGBS
and 30% OPC is recommended. Resistance to chemical attack may be enhanced by
using GGBS in concrete. Resistance to acid attack may be improved through the
use of 70% GGBS. To counter the problem of sulphate and chloride attack 40% to
70% GGBS may be used. There is a general consensus among concrete technologists
that the risk of ASR can be minimised by using at least 50% GGBS. GGBS concrete
is also recommended for use in water retaining structures. Aggressive water can
affect concrete foundations. In such conditions GGBS concrete can perform
better.
Quick
Setting Cement
This
cement as the name indicates sets very early. The early setting property is
brought out by reducing the gypsum content at the time of clinker grinding.
This cement is required to be mixed, placed and compacted very early. It is
used mostly in under water construction where pumping is involved. Use of quick
setting cement in such conditions reduces the pumping time and makes it
economical. Quick setting cement may also find its use in some typical grouting
operations.
Super sulphated cement
is manufactured by grinding together a mixture of 80-85 per cent granulated
slag, 10-15 per cent hard burnt gypsum, and about 5 per cent Portland cement
clinker. The product is ground finer than that of Portland cement. Specific
surface must not be less than 4000 cm2 per gm. The super-sulphated cement is
extensively used in Belgium, where it is known as “ciment metallurgique
sursulfate.” In France, it is known as “ciment sursulfate”.
This
cement is rather more sensitive to deterioration during storage than Portland
cement.
Super-sulphated
cement has a low heat of hydration of about 40-45 calories/gm at 7 days and
45-50 at 28 days. This cement has high sulphate resistance. Because of this
property this cement is particularly recommended for use in foundation, where
chemically aggressive conditions exist. As super-sulphated cement has more
resistance than Portland blast furnace slag cement to attack by sea water, it
is also used in the marine works. Other areas where super-sulphated cement is
recommended include the fabrication of reinforced concrete pipes which are
likely to be buried in sulphate bearing soils. The substitution of granulated
slag is responsible for better resistance to sulphate attack.
Super-sulphated
cement, like high alumina cement, combines with more water on hydration than
Portland cements. Wet curing for not less than 3 days after casting is
essential as the premature drying out results in an undesirable or powdery
surface layer. When we use super sulphated cement the water/cement ratio should
not be less than 0.5. A mix leaner than about 1:6 is also not recommended.
Low
Heat Cement (IS 12600-1989)
It
is well known that hydration of cement is an exothermic action which produces
large quantity of heat during hydration. This aspect has been discussed in
detail in Chapter 1. Formation of cracks in large body of concrete due to heat
of hydration has focussed the attention of the concrete technologists to
produce a kind of cement which produces less heat or the same amount of heat,
at a low rate during the hydration process. Cement having this property was
developed in U.S.A. during 1930 for use in mass concrete
construction,
such as dams, where temperature rise by the heat of hydration can become
excessively large. A Law heat cement is made use of in
low-heat
evolution is achieved by reducing the contents construction of massive dams.
of
C3S and C3A which are the compounds evolving the maximum heat of hydration and
increasing C2S. A reduction of temperature will retard the chemical action of
hardening and so further restrict the rate of evolution of heat. The rate of
evolution of heat will, therefore, be less and evolution of heat will extend
over a longer period.
Therefore,
the feature of low-heat cement is a slow rate of gain of strength. But the
ultimate strength of low-heat cement is the same as that of ordinary Portland
cement. As per the Indian Standard Specification the heat of hydration of
low-heat Portland cement shall be as follows: 7 days — not more than 65
calories per gm.
28
days — not more than 75 calories per gm.
The
specific surface of low heat cement as found out by air-permeability method is
not less than 3200 sq. cm/gm. The 7 days strength of low heat cement is not
less than 16 MPa in contrast to 22 MPa in the case of ordinary Portland cement.
Other properties, such as setting time and soundness are same as that of
ordinary Portland cement.
Portland Pozzolana Cement (IS 1489–1991)
The
history of pozzolanic material goes back to Roman’s time. The descriptions and
details of pozzolanic material will be dealt separately under the chapter
‘Admixtures’. However a brief description is given below.
Portland
Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with
10 to 25 per cent of pozzolanic material (as per the latest amendment, it is 15
to 35%).
A
pozzolanic material is essentially a silicious or aluminous material which
while in itself possessing no cementitious properties, which will, in finely
divided form and in the presence of water, react with calcium hydroxide,
liberated in the hydration process, at ordinary temperature, to form compounds
possessing cementitious properties. The pozzolanic materials generally used for
manufacture of PPC are calcined clay (IS 1489 part 2 of 1991) or fly ash (IS
1489
part I of 1991). Fly ash is a waste material, generated in the thermal power
station, when powdered coal is used as a fuel. These are collected in the
electrostatic precipitator. (It is called pulverised fuel ash in UK). More
information on fly ash as a mineral admixture is given in chapter 5.
It
may be recalled that calcium silicates produce considerable quantities of
calcium hydroxide, which is by and large a useless material from the point of
view of strength or durability. If such useless mass could be converted into a
useful cementitious product, it considerably improves quality of concrete. The
use of fly ash performs such a role. The pozzolanic action is shown below:
Calcium
hydroxide + Pozzolana + water → C – S – H (gel) Portland pozzolana cement produces
less heat of hydration and offers greater resistance to the attack of
aggressive waters than ordinary Portland cement. Moreover, it reduces the
leaching of calcium hydroxide when used in hydraulic structures. It is
particularly useful in marine and hydraulic construction and other mass
concrete constructions. Portland pozzolana cement can generally be used where
ordinary Portland cement is usable. However, it is important to appreciate that
the addition of pozzolana does not contribute to the strength at early ages.
Strengths similar to those of ordinary Portland cement can be expected in
general only at later ages provided the concrete is cured under moist
conditions for a sufficient period.
Status
of PPC in India:
Over
60 million tones of fly ash is generated from over 75 thermal power stations.
But the qualities of such fly ash are generally not satisfactory to be used in
PPC. In western countries fly ash generated in thermal power plants are further
processed to render it fit for using in PPC. Because of the poor quality of fly
ash, lack of awareness and fear psychics on the part of users, PPC is not
popular. In India only 19% of total cement production is PPC.
(1998-1999)
and about 10% is slag cement. Government of India has set up an organisation
called Fly Ash mission to promote the use of fly ash as mineral admixture or in
manufacturing PPC. It has been realised by all experts in the world that more
and more blended cement has to be used for sustainable development of any
country.
Due
to the shortage of electrical power, many cement factories have their own
dedicated thermal power plant. They use their own fly ash for manufacturing
PPC. As they know the importance of the qualities of fly ash, they take
particular care to produce fly ash of good qualities to be used in PPC. The PPC
produced by such cement plant is of superior quality. The chemical and physical
qualities of properties of such PPC show much superior values than what is
prescribed in BIS standard. The physical and chemical properties of PPC as
given in IS: 1489 (part-I) 1991 is given in table 2.5
Birla Plus, Suraksha, Silicate
Cement, Birla Bonus are some of the brand names of PPC in India.
Grading
of PPC:
In
many countries, PPC is graded like OPC depending upon their compressive
strength at 28 days. In India, so far PPC is considered equivalent to 33 grade
OPC, strengthwise, although some brand of PPC is as good as even 53 grade OPC.
Many cement manufacturers have requested BIS for grading of PPC just like
grading of OPC. They have also requested for upper limits of fly ash content
from 25% to 35%. Recently BIS has increased the fly ash content in PPC from
10–25% to 15–35%.
Application:
Portland
pozzolana cement can be used in all situations where OPC is used except where
high early strength is of special requirement. As PPC needs enough moisture for
sustained pozzolanic activity, a little longer curing is desirable. Use of PPC
would be particularly suitable for the following situations:
(a) For hydraulic structures;
(b ) For mass concrete structures
like dam, bridge piers and thick foundation; (c ) For marine structures;
(d ) For sewers and sewage disposal
works etc.
Air-Entraining
Cement
Air-entraining cement is not covered
by Indian Standard so far. This cement is made by mixing a small amount of an
air-entraining agent with ordinary Portland cement clinker at the time of
grinding. The following types of air-entraining agents could be used: (a)
Alkali salts of wood resins.
(b ) Synthetic detergents of the
alkyl-aryl sulphonate type.
(c ) Calcium lignosulphate derived
from the sulphite process in paper making.
(d) Calcium salts of glues and other
proteins obtained in the treatment of animal hides.
"
Concrete
Technology These agents in powder, or in liquid forms are added to the extent
of 0.025–0.1 per cent by weight of cement clinker. There are other additives
including animal and vegetable fats, oil and their acids could be used. Wetting
agents, aluminium powder, hydrogen peroxide could also be used. Air-entraining
cement will produce at the time of mixing, tough, tiny, discrete
non-coalesceing air bubbles in the body of the concrete which will modify the
properties of plastic concrete with respect to workability, segregation and
bleeding. It will modify the properties of hardened concrete with respect to
its resistance to frost action. Air-entraining agent can also be added at the
time of mixing ordinary Portland cement with rest of the ingredients. More
about this will be dealt under the chapter “Admixtures.”
Coloured Cement (White Cement IS 8042–1989)
For manufacturing various coloured cements either white cement or grey Portland
cement is used as a base. The use of white cement as a base is costly. With the
use of grey cement only red or brown cement can be produced.
Coloured
cement consists of Portland cement with 5-10 per cent of pigment. The pigment
cannot be satisfactorily distributed throughout the cement by mixing, and
hence, it is usual to grind the cement and pigment together. The properties
required of a pigment to be used for coloured cement are the durability of
colour under exposure to light and weather, a fine state of division, a
chemical composition such that the pigment is neither effected by the cement
nor detrimental to it, and the absence of soluble salts.
The
process of manufacture of white Portland cement is nearly same as OPC. As the
raw materials, particularity the kind of limestone required for manufacturing
white cement is only available around Jodhpur in Rajasthan, two famous brands
of white cement namely Birla White and J.K. White Cements are manufactured near
Jodhpur. The raw materials used are high purity limestone (96% CaCo3 and less
than 0.07% iron oxide). The other raw materials are china clay with iron content
of about 0.72 to 0.8%, silica sand, flourspar as flux and selenite as retarder.
The fuels used are refined furnace oil (RFO) or gas. Sea shells and coral can
also be used as raw materials for production of white cement.
The
properties of white cement is nearly same as OPC. Generally white cement is
ground finer than grey cement. Whiteness of white cement as measured by ISI
scale shall not be less than 70%. Whiteness can also be measured by Hunters
Scale. The value as measured by Hunters scale is generally 90%. The strength of
white cement is much higher than what is stated in IS code 8042 of 1989.
Hydrophobic
cement (IS 8043-1991)
Hydrophobic
cement is obtained by grinding ordinary Portland cement clinker with water
repellant film-forming substance such as oleic acid, and stearic acid. The
water-repellant film formed around each grain of cement, reduces the rate of
deterioration of the cement during long storage, transport, or under
unfavourable conditions. The film is broken out when the cement and aggregate
are mixed together at the mixer exposing the cement particles for normal
hydration. The film forming water-repellant material will entrain certain
amount of air in the body of the concrete which incidentally will improve the
workability of concrete. In India certain places such as Assam, Shillong etc.,
get plenty of rainfall in the rainy season had have high humidity in other seasons.
The transportation and storage of cement in such places cause deterioration in
the quality of cement. In such far off places with poor communication system,
cement perforce requires to be stored for long time. Ordinary cement gets
deteriorated and loses some if its strength, whereas the hydrophobic cement
which does not lose strength is an answer for such situations.
The properties of
hydrophobic cement is nearly the same as that ordinary Portland cement except
that it entrains a small quantity of air bubbles. The hydrophobic cement is
made actually from ordinary Portland cement clinker. After grinding, the cement
particle is sprayed in one direction and film forming materials such as oleic
acid, or stearic acid, or pentachlorophenol, or calcium oleate are sprayed from
another direction such that every particle of cement is coated with a very fine
film of this water repellant material which protects them from the bad effect
of moisture during storage and transportation. The cost of this cement is nominally
higher than ordinary Portland cement.
Masonry
Cement (IS 3466 : 1988)
Ordinary
cement mortar, though good when compared to lime mortar with respect to " Concrete
Technology strength and setting properties, is inferior to lime mortar with
respect to workability, water-retentivity, shrinkage property and
extensibility.
Masonry
cement is a type of cement which is particularly made with such combination of
materials, which when used for making mortar, incorporates all the good
properties of lime mortar and discards all the not so ideal properties of
cement mortar. This kind of cement is mostly used, as the name indicates, for
masonry construction. It contains certain amount of air-entraining agent and
mineral admixtures to improve the plasticity and water retentivity.
Expansive
Cement
Concrete
made with ordinary Portland cement shrinks while setting due to loss of free
water. Concrete also shrinks continuously for long time. This is known as
drying shrinkage.
Cement
used for grouting anchor bolts or grouting machine foundations or the cement
used in grouting the prestress concrete ducts, if shrinks, the purpose for
which the grout is used will be to some extent defeated. There has been a
search for such type of cement which will not shrink while hardening and
thereafter. As a matter of fact, a slight expansion with time will prove to be
advantageous for grouting purpose. This type of cement which suffers no overall
change in volume on drying is known as expansive cement. Cement of this type
has been developed by using an expanding agent and a stabilizer very carefully.
Proper material and controlled proportioning are necessary in order to obtain
the desired expansion.
Generally,
about 8-20 parts of the sulphoaluminate clinker are mixed with 100 parts of the
Portland cement and 15 parts of the stabilizer. Since expansion takes place
only so long as concrete is moist, curing must be carefully controlled. The use
of expanding cement requires skill and experience.
One
type of expansive cement is known as shrinkage compensating cement. This cement
when used in concrete, with restrained expansion, induces compressive stresses
which approximately offset the tensile stress induced by shrinkage. Another
similar type of cement is known as Self Stressing cement. This cement when used
in concrete induces significant compressive stresses after the drying shrinkage
has occurred. The induced compressive stresses not only compensate the
shrinkage but also give some sort of prestressing effects in the tensile zone
of a flexural member.
IRS-T
40 Special Grade Cement
IRS-T-40
special grade cement is manufactured as per specification laid down by ministry
of Railways under IRS- T40: 1985. It is a very finely ground cement with high
C3S content designed to develop high early strength required for manufacture of
concrete sleeper for Indian Railways. This cement can also be used with
advantage for other applications where high early strength concrete is
required. This cement can be used for prestressed concrete elements, high rise buildings, high strength IRS-T 40
special grade cement was originally made for concrete. manufacturing concrete sleeper for railway
line.
Oil-Well
Cement (IS 8229-1986)
Oil-wells
are drilled through stratified sedimentary rocks through a great depth in
search of oil. It is likely that if oil is struck, oil or gas may escape
through the space between the steel casing and rock formation. Cement slurry is
used to seal off the annular space between steel casing and rock strata and
also to seal off any other fissures or cavities in the sedimentary rock layer.
The cement slurry has to be pumped into position, at considerable depth where
the prevailing temperature may be upto 175°C. The pressure required may go upto
1300 kg/cm2.
The
slurry should remain sufficiently mobile to be able to flow under these
conditions for periods upto several hours and then hardened fairly rapidly. It
may also have to resist corrosive conditions from sulphur gases or waters
containing dissolved salts. The type of cement suitable for the above
conditions is known as Oil-well cement. The desired properties of Oil-well
cement can be obtained in two ways: by adjusting the compound composition of
cement or by adding retarders to ordinary Portland cement. Many admixtures have
been patented as retarders. The commonest agents are starches or cellulose
products or acids. These retarding agents prevent quick setting and retains the
slurry in mobile condition to facilitate penetration to all fissures and
cavities. Sometimes workability agents are also added to this cement to
increase the mobility.
Rediset
Cement
Acclerating
the setting and hardening of concrete by the use of admixtures is a common
knowledge. Calcium chloride, lignosulfonates, and cellulose products form the
base of some of admixtures. The limitations on the use of admixtures and the
factors influencing the end properties are also fairly well known.
High
alumina cement, though good for early strengths, shows retrogression of
strength when exposed to hot and humid conditions. A new product was needed for
use in the precast concrete industry, for rapid repairs of concrete roads and
pavements, and slip-forming.
In
brief, for all jobs where the time and strength relationship was important. In
the PCA laboratories of USA, investigations were conducted for developing a
cement which could yield high strengths in a matter of hours, without showing
any retrogression. Regset cement was the result of investigation. Associated
Cement Company of India have developed an equivalent cement by name “REDISET”
Cement.
High
Alumina Cement (IS 6452 : 1989)
High
alumina cement is obtained by fusing or sintering a mixture, in suitable
proportions, of alumina and calcareous materials and grinding the resultant
product to a fine powder. The raw materials used for the manufacture of high
alumina cement are limestone and bauxite.
These
raw materials with the required proportion of coke were charged into the furnace.
The furnace is fired with pulverised coal or oil with a hot air blast. The
fusion takes place at a temperature of about 1550-1600°C. The cement is
maintained in a liquid state in the furnace.
Afterwards
the molten cement is run into moulds and cooled. These castings are known as
pigs. After cooling the cement mass resembles a dark, fine gey compact rock
resembling the structure and hardeness of basalt rock.
The
pigs of fused cement, after cooling are crushed and then ground in tube mills
to a finess of about 3000 sq. cm/gm.
Hydration
of High Alumina Cement
The
important reaction during the setting of the high alumina cement (HAC) is the
formation of monocalcium aluminate decahydrate (CAH10), dicalcium aluminate
octahydrate (C2 AH8) and alumina gel (AH n).
These aluminates give high strength to HAC concrete but they are metastable and
at normal temperature convert gradually to tricalcium alumina hexahydrate
(C3AH6) and gibbsite which are more stable. The change in composition is
accompanised by a loss of strength and by a change in crystal form from
hexagonal to cubical form with the release of water which results in increased
porosity of concrete. The precise manner in which these changes take place
depends on the temperature, water/cement ratio and chemical environment.
The
change in composition accompanied by loss of strength and change in crystal
form from hexagonal to cubic shape is known as conversion.
Experimental
evidence suggests that in the important reaction of the conversion from CAH10
to C3AH6 and alumina hydrate, temperature effects the decomposition. The higher
the temperature, the faster the rate of conversion. Experimental studies have
also shown that the Types of Cement "
It should be noted that
this reaction liberates all the water needed for the conversion process to
continue. The conversion reaction will result in a reduction in volume of the
solids and an increase in the porosity, since the overall dimensions of
specimens of cement paste or concrete remain sensibly constant.
High Alumina Cement Concrete
The
use of high alumina cement concrete commenced in the U.K. in 1925 following its
introduction in France where it had been developed earlier to make concrete
resistant to chemical attack, particularly in marine conditions. The capability
of this concrete to develop a high early strength offers advantages in
structural use. However, its high cost prevented extensive use of high alumina
cement for structural purposes. All the same during 1930’s many structures were
built in European countries using high alumina cement. Following the collapse
of two roof beams in a school at Stepney in U.K. in February 1974, the Building
Research Establishment (BRE) of U.K. started field studies and laboratory tests
to establish the degree of risk likely in buildings with precast prestressed
concrete beams made with high alumina cement. The results of the BRE
investigations are summarised below: 1.
Measurements
of the degree of conversion of the concrete used in the buildings indicated
that high alumina cement concrete reaches a high level of conversion within a
few years. The concrete specimens cut from beams indicated that some concrete
suffered substantial loss of strength when compared to one day strength on
which the design was earlier based
Very
High Strength Cement
(a) Macro-defect-free cements (MDF)2.4. The
engineering of a new class of high strength cement called Macro-defect-free
(MDF) cements is an innovation. MDF refers to the absence of relatively large
voids or defects which are usually present in conventional mixed cement pastes
because of entrapped air and inadequate dispersion. Such voids and defects
limit the strength. In the MDF process 4-7% of one of several water-soluble
polymers (such as hydroxypropylmethyle cellulose, polyacrylamide of hydrolysed
polyvinylacetate) is added as rheological aid to permit cement to be mixed with
very small amount of water. Control of particle size distribution was also
considered important for generating the strength. At final processing stage
entrapped air is removed by applying a modest pressure of 5 MPa.
With
this process a strength of 300 MPa for calcium aluminate system and 150 MPa for
Portland cement system can be achieved.
(b) Densely Packed System (DSP). New materials termed DSP (Densified system
containing homegeneously arranged ultre-fine particles) is yet another
innovation in the field of high strength cement. Normal Portland cement and
ultra-fine silica fume are mixed. The size of cement particles may very from
0.5 to 100µ and that of silica fume varies from 0.005 to 0.5µ.
Silica
fume is generally added from 5 to 25 %. A compressive strength of 270 MPa have
been achieved with silica fume substituted paste.
The
formation of typical DSP is schematically represented in Fig. 2.4.
(c) Pressure Densification and Warm
Pressing. For decades uncertainties
existed regarding the theoretical strength of hydrated cement paste. Before 1970,
the potential strength of cement paste at theoretical density (What T.C. Powers
called “intrinsic strength”) had never been achieved because of considerable
porosity (20 to 30% or more) always remain ofter completing hydration of
cement. A new approach has ben developed for achieving very high strength by a
method called “Warm Pressing” (applying heat and pressure simultaneously) to
cement paste. Some modest increase in strength was achieved by application of
pressure alone.
Compressive
strength as much as 650 MPa and tensile strength up to 68 MPa have been
obtained by warm pressing Portland and calcium aluminate cements. Enormous
increases in strength resulted from the removal of most of the porosity and
generation of very homogeneous, fine micro-structures with the porosities as
low as 1.7%.
(d)
High Early Strength Cement. Development of high
early strength becomes an important factor, sometimes, for repair and emergency
work. Research has been carried out in the recent past to develop rapid setting
and hardening cement to give materials of very high early strength.
Lithium
salts have been effectively used as accelerators in high alumina cement. This
has resulted in very high early strength in cement and a marginal reduction in
later strength.
Strength
as high as 4 MPa has been obtained within 1 hour and 27 MPa has been obtained
within 3 hours time and 49 MPa in one day.
(e) Pyrament Cement. Some cement industries in USA have developed a
super high early strength and durable cement called by trade name “Pyrament
Cement”. This product is a blended hydraulic cement. In this cement no
chlorides are added during the manufacturing process. Pyrament cement produces
a high and very early strength of concrete and mortar which can be used for
repair of Air Field Run-ways. In India Associated Cement Company in
collaboration with R & D Engineers, Dighi, Pune have also produced high
early strength cement for rapid repair of airfields.
The
Pyrament cement showed the following strength. Refer Table 2.4.
(f) Magnesium Phosphate Cement (MPC). Magnesium
Phosphate Cement, an advanced cementing material, giving very high early
strength mortar and concrete has been developed by Central Road Research
Institute, New Delhi. This cement can be used for rapid repair of damaged
concrete roads and airfield pavements. This is an important development for
emergency repair of airfields, launching pads, hard standing and road pavements
suffering damage due to enemy bombing and missile attack.
TESTING
OF CEMENT
Testing
of cement can be brought under two categories: (a) Field testing
(b)
Laboratory testing.
Field Testing
It
is sufficient to subject the cement to field tests when it is used for minor
works. The following are the field tests:
(a)
Open the bag and take a good look at the cement. There should not be any
visible lumps. The colour of the cement should normally be greenish grey.
(b)
Thrust your hand into the cement bag. It must give you a cool feeling. There
should not be any lump inside.
(c)
Take a pinch of cement and feel-between the fingers. It should give a smooth
and not a gritty feeling.
(d)
Take a handful of cement and throw it on a bucket full of water, the particles
should float for some time before they sink.
"
Concrete
Technology (e) Take about 100 grams of cement and a small quantity of water and
make a stiff paste.
From
the stiff paste, pat a cake with sharp edges. Put it on a glass plate and
slowly take it under water in a bucket. See that the shape of the cake is not
disturbed while taking it down to the bottom of the bucket. After 24 hours the
cake should retain its original shape and at the same time it should also set
and attain some strength.
If
a sample of cement satisfies the above field tests it may be concluded that the
cement is not bad. The above tests do not really indicate that the cement is
really good for important works. For using cement in important and major works
it is incumbent on the part of the user to test the cement in the laboratory to
confirm the requirements of the Indian Standard specifications with respect to
its physical and chemical properties. No doubt, such confirmations will have
been done at the factory laboratory before the production comes out from the
factory. But the cement may go bad during transportation and storage prior to
its use in works. The following tests are usually conducted in the laboratory.
(a)
Fineness test.
(b)
Setting time test.
(c)
Strength test.
(d
) Soundness test.
(e)
Heat of hydration test.
(f
) Chemical composition test.
Fineness Test
The
fineness of cement has an important bearing on the rate of hydration and hence
on the rate of gain of strength and also on the rate of evolution of heat.
Finer cement offers a greater surface area for hydration and hence faster the
development of strength, The fineness of grinding has increased over the years.
But now it has got nearly stabilised. Different cements are ground to different
fineness. The disadvantages of fine grinding is that it is susceptible to
air-set and early deterioration. Maximum number of particles in a sample of
cement should have a size less than about 100 microns. The smallest particle
may have a size of about 1.5 microns. By and large an average size of the
cement particles may be taken as about 10 micron. The particle size fraction
below 3 microns has been found to have the predominant effect on the strength
at one day while 3-25 micron fraction has a major influence on the 28 days
strength. Increase in fineness of cement is also found to increase the drying
shrinkage of concrete. In commercial cement it is suggested that there should
be about 25-30 per cent of particles of less than 7 micron in size.
Fineness of cement is
tested in two ways :
(a)
By seiving.
(b)
By determination of specific surface (total surface area of all the particles
in one gram of cement) by air-premeability appartus. Expressed as cm2/gm or
m2/kg. Generally Blaine Airpermeability appartus is used.
Sieve
Test
Weigh
correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90
microns).
Break down the air-set lumps in the sample with fingers. Continuously sieve the
sample giving circular and vertical motion for a period of 15 minutes.
Mechanical sieving devices may also be used. Weigh the residue left on the
sieve. This weight shall not exceed 10% for ordinary cement. Sieve test is
rarely used.
Air Permeability Method
This
method of test covers the procedure for determining the fineness of cement as
represented by specific surface expressed as total surface area in sq. cm/gm.
of cement. It is also expressed in m2/kg. Lea and Nurse Air Permeability
Appartus is shown in Fig. 2.6. This appartus can be used for measuring the
specific surface of cement. The principle is based on the relation between the
flow of air through the cement bed and the surface area of the particles
comprising the cement bed. From this the surface area per unit weight of the
body material can be related to the permeability of a bed of a given porosity.
The cement bed in the permeability cell is 1 cm. high and 2.5 cm. in diameter.
Knowing the density of cement the weight required to make a cement bed of
porosity of 0.475 can be calculated. This quantity of cement is placed in the
permeability cell in a standard manner. Slowly pass on air 50 " Concrete Technology through
the cement bed at a constant velocity. Adjust the rate of air flow until the
flowmeter shows a difference in level of 30-50 cm. Read the difference in level
(h1) of the manometer and the difference in level (h2) of the flowmeter. Repeat
these observations to ensure that steady conditions have been obtained as shown
by a constant value of h1/h2. Specific surface Sw is calculated from the
following formula:
Standard
Consistency Test
For
finding out initial setting time, final setting time and soundness of cement,
and strength a parameter known as standard consistency has to be used. It is
pertinent at this stage to describe the procedure of conducting standard
consistency test. The standard consistency of a cement paste is defined as that
consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm
length to penetrate to a depth of 33-35 mm from the top of the mould shown in
Fig. 2.8. The appartus is called Vicat Appartus. This appartus is used to find
out the percentage of water required to produce a cement paste of standard
consistency.
The
standard consistency of the cement paste is some time called normal consistency
(CPNC).
The
following procedures is adopted to find out standard consistency. Take about
500
gms
of cement and prepare a paste with a weighed quantity of water (say 24 per cent
by weight of cement) for the first trial. The paste must be prepared in a
standard manner and filled into the Vicat mould within 3-5 minutes. After
completely filling the mould, shake the mould to expel air. A standard plunger,
10 mm diameter, 50 mm long is attached and brought down to touch the surface of
the paste in the test block and quickly released allowing it to sink into the paste
by its own weight. Take the reading by noting the depth of penetration of the
plunger. Conduct a 2nd trial (say with 25 per cent of water) and find out the
depth of penetration of plunger. Similarly, conduct trials with higher and
higher water/cement ratios till such time the plunger penetrates for a depth of
33-35 mm from the top. That particular percentage of water which allows the
plunger to penetrate only to a depth of 33-35 mm from the top is known as the
percentage of water required to produce a cement paste of standard consistency.
This percentage is usually denoted as ‘P’. The test is required to be conducted
in a constant temperature (27° + 2°C) and constant humidity (90%).
Setting Time Test
An
arbitraty division has been made for the setting time of cement as initial
setting time and final setting time. It is difficult to draw a rigid line
between these two arbitrary divisions. For convenience, initial setting time is
regarded as the time elapsed between the moment that the water is added to the
cement, to the time that the paste starts losing its plasticity. The final
setting time is the time elapsed between the moment the water is added to the
cement, and the time when the paste has completely lost its plasticity and has
attained sufficient firmness to resist certain definite pressure.
In
actual construction dealing with cement paste, mortar or concrete certain time
is required for mixing, transporting, placing, compacting and finishing. During
this time cement paste, mortar, or concrete should be in plastic condition. The
time interval for which the cement products remain in plastic condition is
known as the initial setting time. Normally a minimum of 30 minutes is given
for mixing and handling operations. The constituents and fineness of cement is
maintained in such a way that the concrete remains in plastic condition for
certain minimum time. Once the concrete is placed in the final position,
compacted and finished, it should lose its plasticity in the earliest possible
time so that it is least vulnerable to damages from external destructive
agencies. This time should not be more than 10 hours Concrete Technology which
is often referred to as final setting time. Table 2.5 shows the setting time
for different cements.
The
Vicat Appartus shown in Fig. 2.8 is used for setting time test also. The
following procedure is adopted. Take 500 gm. of cement sample and guage it with
0.85 times the water required to produce cement paste of standard consistency
(0.85 P). The paste shall be guaged and filled into the Vicat mould in
specified manner within 3-5 minutes. Start the stop watch the moment water is
added to the cement.
Initial
Setting Time
Lower
the needle (C) gently and bring it in contact with the surface of the test
block and quickly release. Allow it to penetrate into the test block. In the
beginning, the needle will completely
pierce through the test block. But after some time when the paste starts Vicat
Apparatus and Automatic Vicat Apparatus.
losing its plasticity, the Accessories. needly may penetrate only to a
depth of 33-35 mm from the top. The period elapsing between the time when water
is added to the cement and the time at which the needle penetrates the test
block to a depth equal to 33-35 mm from the top is taken as initial setting
time.
Final Setting Time
Replace
the needle (C) of the Vicat appartus by a circular attachment (F) shown in the
Fig 2.8. The cement shall be considered as finally set when, upon, lowering the
attachment gently cover the surface of the test block, the centre needle makes
an impression, while the circular cutting edge of the attachment fails to do
so. In other words the paste has attained such hardness that the centre needle
does not pierce through the paste more than 0.5 mm.
Strength Test
The
compressive strength of hardened cement is the most important of all the
properties.
Therefore,
it is not surprising that the cement is always tested for its strength at the
laboratory before the cement is used in important works. Strength tests are not
made on neat cement paste because of difficulties of excessive shrinkage and
subsequent cracking of neat cement.
Strength
of cement is indirectly found on cement sand mortar in specific proportions.
The standard sand is used for finding the strength of cement. It shall conform
to IS 650-1991. Take 555 gms of standard sand (Ennore sand), 185 gms of cement
(i.e., ratio of cement to sand is 1:3) in a non-porous enamel tray and mix them
with a trowel for one minute, then add water of quantity P + 3.0 per cent of
combined 4 weight of cement and sand and mix the three ingredients thoroughly
until the mixture is of uniform colour.
The time of mixing should not be less than 3 minutes nor more than 4 minutes.
Immediately after mixing, the mortar is
filled into a cube mould of size 7.06 cm. The area of the face of the cube will
be equal to 50 sq cm. Compact the mortar either by hand compaction in a
standard specified manner or on the vibrating equipment (12000 RPM) for 2
minutes.. Moulding of 70.7 mm Mortar
Cube Vibrating Machine. Keep the
compacted cube in the mould at a temperature of 27°C ± 2°C and at least 90 per
cent relative humidity for 24 hours. Where the facility of standard temperature
and humidity room is not available, the cube may be kept under wet gunny bag to
simulate 90 per cent relative humidity. After 24
hours
the cubes are removed from the mould and immersed in clean fresh water until
taken out for testing.
Three
cubes are tested for compressive strength at the periods mentioned in Table
2.5.
The
periods being reckoned from the completion of vibration. The compressive
strength shall be the average of the strengths of the three cubes for each
period respectively.
It is very important
that the cement after setting shall not undergo any appreciable change of
volume. Certain cements have been found to undergo a large expansion after
setting causing disruption of the set and hardened mass. This will cause
serious difficulties for the durability of structures when such cement is used.
The testing of soundness of cement, to ensure that the cement does not show any
appreciable subsequent expansion is of prime importance.
The
unsoundness in cement is due to the presence of excess of lime than that could
be combined with acidic oxide at the kiln. This is also due to inadequate
burning or insufficiency in fineness of grinding or thorough mixing of raw
materials. It is also likely that too high a proportion of magnesium content or
calcium sulphate content may cause unsoundness in cement. For this reason the
magnesia content allowed in cement is limited to 6 per cent. It can be recalled
that, to prevent flash set, calcium sulphate is added to the clinker while
grinding. The quantity of gypsum added will vary from 3
to
5 per cent depending upon C3A content. If the addition of gypsum is more than
that could be combined with C3A, excess of gypsum will remain in the cement in
free state. This excess of gypsum leads to an expansion and consequent
disruption of the set cement paste.
Unsoundness
in cement is due to excess of lime, excess of magnesia or excessive proportion
of sulphates.
Unsoundness
in cement does not come to surface for a considarable period of time.
Therefore, accelerated tests are required to detect it. There are number of
such tests in common use. The appartus is shown in Fig. 2.9. It consists of a
small split cylinder of spring brass or other suitable metal. It is 30 mm in
diameter and 30 mm high.
On
either side of the split are attached two indicator arms 165 mm long with pointed
ends. Cement is gauged with 0.78 times the water required for standard
consistency (0.78 P), in a standard manner and filled into the mould kept on a
glass plate. The mould is covered on the top Autoclave.
Heat
of Hydration
The
reaction of cement with water is exothermic.
The
reaction liberates a considerable quantity of heat.
This
can be easily observed if a cement is gauged with water and placed in a thermos
flask. Much attention has been paid to the heat evolved during the hydration of
cement in the interior of mass concrete dams. It is estimated that about 120
calories of heat is generated in the hydration of 1 gm. of cement. From this it
can be assessed the total quantum of heat produced in a conservative system
such as the interior of a mass concrete dam. A temperature rise of about 50°C
has been observed. This unduly high temperature
developed
at the interior of a concrete dam causes serious expansion of the body of the
dam and with the subsequent cooling considerable shrinkage takes place Heat of
hydration Apparatus. resulting in serious cracking of concrete.
"
Concrete
Technology The use of lean mix, use of pozzolanic cement, artificial cooling of
constituent materials and incorporation of pipe system in the body of the dam
as the concrete work progresses for circulating cold brine solution through the
pipe system to absorb the heat, are some of the methods adopted to offset the
heat generation in the body of dams due to heat of hydration of cement.
Test
for heat of hydration is essentially required to be carried out for low heat
cement only. This test is carried out over a few days by vaccum flask methods,
or over a longer period in an adiabatic calorimeter. When tested in a standard
manner the heat of hydration of low heat Portland cement shall not be more than
65 cal/gm. at 7 days and 75 cal/g, at 28 days.
Chemical Composition Test
A
fairly detailed discussion has been given earlier regarding the chemical
composition of cement. Both oxide composition and compound composition of
cement have been discussed.
At
this stage it is sufficient to give the limits of chemical requirements. The
Table 2.6 shows the various chemical compositions of all types of cements.
Test
Certificate
Every
cement company is continuously testing the cement manufactured in their
factory.
They
keep a good record of both physical and chemical properties of the cement
manufactured applying a batch number. Batch number indicates date, month and
year.
They
also issue test certificate. Every purchaser is eligible to demand test
certificate.
A
typical test certificate of Birla super 53 grade cement for the week number 35
is given in Table 2.7 for general information.
Some
cement companies also work out the standard deviation and coefficient of
variation for 3 months or 6 months or for one year period subjecting the
various parameters obtained from their test results. Table 2.8 shows the
typical standard deviation for 3 days, 7 days and 28 days strength in respect
of 53 grade cement Birla super. Standard deviation has been worked out for the
whole year from Jan. 99 to Dec. 99.
The
properties of cements, particularly the strength property shown in Table No.
2.5 is tested as per the procedures given by BIS. In different countries cement
is tested as per their own country’s code of practice. There are lot of
variations in the methods of testing of cement with respect to w/c ratio, size
and shape of specimen, material proportion, compacting methods and temperature.
Strength of cement as indicated by one country may not mean the same in another
country. This will present a small problem when export or import of cement from
one country to another country is concerned. Table No. 2.9. Shows the cements
testing procedure and various grades of cement manufactured in some countries.
There is suggestion that all the countries should follow one method recommended
by International standards organisation for testing of cement. If that system
is adopted properties indicated by any one country will mean the same to any
other country.